1. Field of the Invention
The present invention relates to thermosensitive recording materials for forming colored images upon heating. More particularly, the present invention relates to thermosensitive recording materials having a high whiteness, an excellent colored image-retaining stability, due to which the color-developed images are not erased, and a high recording sensitivity. The thermosensitive recording materials of the present invention exhibit a good storage property over a long time, have a high environmental resistance such as a high humidity resistance, a high oil resistance, and a high plasticizer resistance of the recorded images, and have a high recording sensitivity and whiteness. Thus the thermosensitive recording materials of the present invention are usable for image-recording sheets, cash-dispenser sheets, ticket cards, commuting ticket cards, labels, for example, POS labels, cards, for example, prepaid cards, and pass cards.
2. Prior Art
Generally, a thermosensitive recording material comprises a substrate comprising a paper sheet, a synthetic paper sheet or a plastic film, and a thermosensitive colored image-forming layer formed on the substrate and comprising, as principal components, a color-forming component, for example, an electron-donative leuco dye and a color-developing component comprising an organic acid substance, for example, an electron-acceptive phenol compound. The above-mentioned two components react with each other upon heating to form colored images. This type of thermosensitive recording material is disclosed in Japanese Examined Patent Publication (Kokoku) No. 43-4160, Japanese Examined Patent Publication (Kokoku) No. 45-14039, and Japanese Examined Patent Publication (Kokoku) No. 48-27736, and is employed in practice.
The thermosensitive recording material is compact, cheap and is easy to maintain, and thus is used in a broad range of practical applications, for example, outputs of computers, facsimiles, automatic ticket vending machines, printers for scientific and measurement equipments, printer for CRT medical measurements and the like. However, in the conventional thermosensitive recording materials having a substrate sheet coated with a thermosensitive colored image-forming layer comprising a color-forming dye component, a color-developing component and a binder as the active components, the color-forming reaction is reversible and thus it is known that the colored images is erased with the lapse of time. The color-erasure is accelerated when the images are exposed to light, a high humidity atmosphere, or a high temperature atmosphere. The color-erasure is further promoted by immersing in water for long period or by contacting with an oil, for example, salad oil, or with a plasticizer, to such an extent that the images cannot be read or noted.
On the other hand, it is also known that dye type thermosensitive recording materials undergo a color forming reaction upon heating, so if held in a high temperature, high humidity environment, will spontaneously form color (hereafter, so called “spontaneous color formation”) leading to a deterioration of the contrast and difficulty in discriminating the recorded image. Further, thermosensitive recording type parking lot tickets and highway tickets left in motor vehicles in hot summer weather and thermosensitive type POS labels heated by microwave ovens along with food they are attached to in supermarkets or convenience stores suffer from serious spontaneous color formation, making it almost impossible to read the recorded images.
Much technology has been disclosed for suppressing the color-erasure phenomenon using a color-forming system containing a dye including, as a principal component, a normally colorless or light colored lactone cyclic compound. For example, in one attempt, a phenol compound-containing antioxidant is contained in the thermosensitive colored image-forming layer as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-78782, Japanese Unexamined Patent Publication (Kokai) No. 59-167292, Japanese Unexamined Patent Publication (Kokai) No. 59-114096, and Japanese Unexamined Patent Publication (Kokai) No. 59-93387; in another attempt, a protective layer is formed from a hydrophobic polymer emulsion on the thermosensitive colored image-forming layer as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 56-146796; in still another attempt, the thermosensitive colored image-forming layer is coated with an intermediate layer formed from an emulsion of a water-soluble polymeric compound or a hydrophobic polymeric compound, and the intermediate layer is coated by a surface layer formed from a oily lacquer containing, as a resin component, a hydrophobic polymeric compound, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-199189; in still another attempt, a phenol compound-containing color developing agent is employed in combination with an epoxy compound as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-164579; and in further another attempt, a metal salt of a specific salicylic acid derivative is employed as a color-developing agent as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-169681. However, none of these is deemed to exhibit much of an effect of improvement or else, while some degree of improvement is seen, a long time is required for it to be manifested or the effect does not last long or otherwise is not satisfactory. Further, sometimes the improving procedure results in problems such as spontaneous color formation due to high heat or high humidity, a drop in recording sensitivity, etc. A more practical proposal has therefore been awaited for a long time.
As a completely different strategy from the above, technology has been disclosed using a sulfonylurea compound for the color developing agent instead of the conventional organic acid compounds like phenol (see Japanese Unexamined Patent Publication (Kokai) No. 5-147357, p. 2 and Japanese Unexamined Patent Publication (Kokai) No. 5-32601, pp. 2 to 4). The proposal of using a sulfonylurea compound as the color developing agent is extremely epochmaking. It not only creates novel color-forming functional groups, but also succeeds in completely suppressing the phenomenon of erasure of the recorded image of thermosensitive recording materials not only under various environment conditions such as high temperatures and high humidities, but also in contact with oil or plasticizers—which had been considered impossible in the past. Thermosensitive recording materials with an extremely high storage stability of recorded images were, therefore, accomplished.
However, due to the striking improvement in the storage stability of the recorded images of thermosensitive recording materials by sulfonylurea color developing agents, a strong demand has arisen for early improvement of the other problem in thermosensitive recording materials, that is, the spontaneous color formation when held under a high temperature or high humidity and the consequent deterioration of contrast and difficulty in discerning the recorded images. That is, while the proposal for use of sulfonylurea compounds for color developing agents has been effective in improving the storage stability of recorded images, suppression of the spontaneous color formation occurring under more severe environmental conditions which are sought at the present time has not yet necessarily been achieved.
Further, while sulfonylurea based color developing agents similar in chemical structure to the present invention (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2000-355578, pp. 2 to 8) which are effective in improving the storage stability of recorded images have been proposed, the suppression of the spontaneous color formation under harsh environmental conditions which are currently sought cannot be said to be sufficient.
Thermosensitive recording materials form colors due to heating as part of their inherent system, so spontaneous color formation under high temperature environments is unavoidable, but improvements in color developing agents and sensitizers have made it possible to keep this to a lower level (for example, see Japanese Unexamined Patent Publication (Kokai) No. 6-1069, p. 2). In recent years, however, performance has been sought from thermosensitive paper enabling resistance to spontaneous color formation, and consequently, good contract and legibility even when, for example, thermosensitive recording type-parking or highway tickets using it are left in cars in hot summer weather or when thermosensitive recording type POS labels are heated by microwave ovens along with the food they are attached to in supermarkets or convenience stores. A great turning point has been reached in thermosensitive recording materials.
As one proposal to meet this demand, there is the method of using compounds with relatively high molecular weights for the color developing agent. This proposal, however, also results in a low stability of recorded images and is not sufficient for practical use (for example, see Japanese Unexamined Patent Publication (Kokai) No. 8-333329, p. 2).
In this way, no thermosensitive recording materials offering both storage stability of the recorded images and suppression of spontaneous color formation under harsher environments, and further having both practical and sufficient basic performance have yet been proposed.